Fluidized-bed reactors have been used in exothermic processes, such as the combustion of carbonaceous materials or the roasting of sulfide ores (see British Pat. No. 784 595 and J. R. Grace, Fluidization and its Application to Coal Treatment and Allied Processes, AIChE Symposium Series 141, vol. 70, (1974), pages 21 - 26; D. L. Keairns et al., Design of a Fluidized Bed Combustion Boiler for Industrial Steam Generation, AIChE Symposium Series 126, vol. 68 (1972), pages 259 - 266; Australian Pat. No. 164 429).
The use of known methods for the combustion of carbonaceous materials involves the disadvantages that the bed must be comparatively low in order to keep the pressure drop within reasonable limits, that the presence of cooling surfaces (internal fixtures) in the lower part of the reactor space disturbs the transverse mixing of the solids in the fluidized bed so that inhomogeneities of temperature (overheating, crusts) occur, and that the operation of the reactor cannot be perfectly adapted to the output actually required, which often fluctuates. This adaptation can be accomplished virtually only by a decrease of temperature, although such decrease involves less satisfactory combustion and fluidization conditions, or by a shutdown of individual reactor units.
Equipment is known in which the temperature in the fluidized bed can be increased or decreased in that solids withdrawn from the fluidized bed are heated or cooled in a second fluidized bed and are then recycled to the first fluidized bed through suitable lock chambers and overflow means (U.S. Pat. No. 3 921 590).
For instance, a combustion process can be carried out in the first fluidized bed and fluidized solids can be fed from the first fluidized bed to the second to be cooled therein and subsequently are recycled to the first bed. The use of such apparatus involves the disadvantage that the pressure drop in the fluidized bed used for the combustion process is high owing to the condition of this bed. For this reason, the fluidizing gases must be compressed to a high pressure with a high expenditure of energy.
In addition the fluidizing gases used to cool the solids withdrawn from the fluidized bed do not contribute to the economy of the process but only increase the rate at which exhaust gases are produced in the combustion process.
In the roasting sulfide ores in a fluidized bed in accordance with Australian Pat. No. 164 429, inhomogeneities of temperature will inevitably be caused by the spontaneous combustion of the sulfur in the presence of the total quantity of oxygen which is required. This results in calcines which have undesired metallurgical properties.